Method and apparatus for indicating cathodic protection



J. R. COWLES Aug. 2o, 1957 METHOD AND APPARATUS FOR lNDICATING CATHODIC PROTECTION Filed sept. 17. 195s IN V EN TOR. e/lzzes'owles A T raw/VE)l ilnited States Patent O METHOD AND APPARATUS FOR INDICATING CATHODIC PROTECTION This invention relates to a device for measuring and indicating cathodic protection with special reference to pipes and the like buried underground.

Any metallic structure, such as a steel pipe, lead cable sheath or a storage tank, when placed in an electrolyte such as wet soil, sets up a flow of current from one portion to another or to other metalli-c bodies 'analog-ous to local action in a battery. Stray currents from nearby electric railways or grounded power lines contributeto this action. Small quantities of the metal Iof the pipe or sheath, as a result of this action, go into solution at the anode where the current leaves the structure. This causes pitting, deterioration, and may even cause the ultimate `failure of the structure. One of the more efficient and economical ways of counteracting this corrosion in buried structures is obtained by causing a direct current to ow from an outside source in the opposite directi-on to make the buried structure cathodic instead of anodic.

An example of eathodic protection, :as used in the above described manner, would be to employ a -full wave dry-plate rectifier and to connect the positive terminal .of the rectifier to buried metal scrap or plates and the negative terminal to the structure requiring protection.

The concept of cathodic protection has also been applied to prevent rust in water tanks, stand pipes, and hot wells. The use of cathodic protection has become sufficiently wide spread in recent years, that a large amount of experiments and practical data is available to persons desiring information concerning cathodic protection.

In order to check the oathodic protection of an underground structure, it has been `the practice to use, as a standard, a copper sulfate electrode in combination with a volt meter for structures buried under earth, and a calomel electrode for structures under sea water. The electrode is usually of the type known as a half-cell and the apparatus is generally portable. It is, however, possible to use electrodes of metals -or materials other than copper sulfate or calomel, however, in order to use these metals or materials intelligibly, the information and data obtained must be interpolated on the basis of a standard electrode. For this reason, it has been exceedingly difficult for cathodic protection Ito be checked by other than skilled personnel who are fully cognizant of the electrode being used and their characteristics with respect to standard electrodes.

It is, accordingly, an object of the present invention to provide a method and device for checking cathodic protection which can be carried out by unskilled personnel.

It is a further object of the present invention to provide a method and device for checking cathodic protection of buried structures which will enable data to be directly obtained without regard to the type of electrode being employed :and without the necessity of interpolating the data inthe light of standard electrodes.

It is a still further object of the present invention to provide a device for checking cathodic protection which will be simpler and more efficient than methods and devices that have heretofore been advanced.

Other `objects and advantages Iof the present invention will become readily .apparent from the `following descrip; tion when ,taken in conjunction with the appended drawing in which the single figure shows the method and `device of the present invention being used to check cathodic protection of a buried structure.

A normal reading between `a steel pipe not involved by any electrolysis or cathodic protection and a standard copper sulfate electrode is approximately 0.55 volt. When cathodic protection is effectively :applied to .a buried steel pipe, this voltage will rise to values from 0.8 volt t-o about 2.5 volts.

lt is to be understood that the above is merely representative and in no way limits the scope of this invention.

Referring to the drawing, y10 indicates a pipe buried underneath the surface of the earth 11. In order to afford cathodic protection, a rectier 12 is connected with its negative terminal 13 attached to the pipe 10 and its positive terminal 14 grounded at 15. The other terminals of the rectifier are connected through a transformer 16 to a suitable source of alternating current as is conventional.

The method and device for checking the cathodic protection on the pipe -10 consists of an electrode 20' buried in the earth 11 and a novel arrangement for a volt meter. This volt meter consists of a fixed multiplier resistor 21 and a variable multiplier resistor 22. These two resistors 21 and 22 are connected in series -to one side of the meter movement 23. To the other side of the meter movement is connected a mercury battery 24 in parallel with a voltage divider 25. A double-pole single-throw switch 26 is connected in circuit with the mercury battery 24 and voltage `divider' 2S. One lead 27 of the volt meter is connected to a contact 28 permanently fixed at the surface of the earth 11, which in turn is connected by a lead line 29 to the pipe 10. The other lead 30 of the volt meter is connected to a contact 31 also permanently fixed, -which in turn is connected to the permanently buried electrode 20.y

The' electrode 20 can be composed of any metal in any form such as a rod or it 4can be a base having sprayed or plated film of metal. The metal can be, if desired, coated with a cement -of the type used in dentistry. If desired, a non-metallic conductor such as graphite or carbon, either as rods or slabs, which if desired, can be treated with waxes, chemicals, and metal films.

By means of the particular structure of the volt meter, it is possible to use .any of the above mentioned metals for the electrode 20 and still obtain direct readings from the meter movement 23 in terms of a standard electrode. By having the multiplier resistor 22 variable, it is possible to make the meter movement 23 agree with a portable meter and standard half-cell electrode regardless of the differences of voltage between the standard half-cell electrode and electrode 20. These differences may result lfrom the materials having different positions in the electromotive force series. The operability of resistor -2-2 is limited, however, to the elimination of small ldifferences in cases where the materials are relatively close in `the electromotive force series. Further, by having the multiplier resistor 22 variable, any differences due to structure in the two meters can .be eliminated and also the meter movement 23 can be compensated for surrounding steel or magnetic effects for all points on the scale. It is desirable to select multipliers `which are made of fine wires .and thus .a certain amount of lightning protection for the meter movement 23 can be had. The switch 26 functions to protect the meter from lightning damage and facilitates adjustment.

If the difference in galvanic voltage between electrode 20 and the reference standard electrode is relatively great, the mercury battery 24 and the voltage divider 25 can be adjusted with respect to the meter movement Z3 so that its indications will be as though the electrode 20 were a standard electrode.

The electrode 29 can be surrounded by soil or earth as shown, or suitable chemicals commonly called backfill, which may consist of powdered or crystalline chemicals, a slurry `or cements of types other than those used in building construction.

From the above description it can be seen that persons unskilled in cathodic protection, can simply, by reading the meter movement 23, determine whether or not the pipe 10 has cathodic protection without knowing the characteristics of the electrode 20.

On well coated insulated structures, the connection to the structure (lead 29) can be at the point of least structure to soil potential and thus rthe meter movement 23 will indicate the degree of protection to the structure as a whole. lt the cathodic protection is secured by an inaccessible source of electricity, such as a rectiiier remotely located, which is impractical to check directly, the meter movement 23 will alford an indication as to whether the rectier is performing its function properly.

It will be appreciated at this time that the difference between the standard electrode and any other electrode is a constant. Thus, for example, a suitable volt meter connected between a zinc electrode and a standard copper sulfate electrode will read 1.1 volts. The meter of the present invention can be arranged with a scale that centers or zeros on this constant difference and accordingly the zero in such a case would be at 1.1 volts. With such a construction, the needle `of the meter can deflect either to the left or the right from its new zero vthus enabling the meter to provide an indication of cathodic protection irrespective of lthe polarity of the binding post on the meter to which the electrode is attached.

While this invention has been shown and -described in a specic embodiment, nevertheless various changes and modifications obvious to one skilled in the art yare within the spirit, scope and contemplation of the present invention.

What is claimed is:

1. A method for determining cathodic protection of structures located in an electrolyte such as wet soil in terms of a standard reference electrode that comprises 1. impressing a voltage on said structure andthe surrounding soil in a direction to make said structure cathodic with respect to the surrounding soil, establishing an electrode Within the soil in proximity with said structure, connecting the electrode to the structure, for measuring voltage between the electrode .and the structure, and obtaining directly voltage readings between said electrode and said structure in terms of the standard reference electrode.

2. Apparatus yfor checking cathodic protection of la structure located in an electrolyte that comprises an electrode adapted to be placed in said electrolyte and 4a volt meter adapted to be connected with one lead to said electrode and adapted to be connected with its other lead to said structure, said volt meter characterized by a meter movement, multiplier yresistor means including a variable component, and means to vary the voltage drop between said electrode and said structure.

3. Apparatus as defined in claim 2 wherein said means to vary the voltage drop consists of a mercury battery in parallel with a voltage divider.

4. Apparatus as defined in claim 2 wherein said electrode is selected from the class consisting of metallic and non-metallic conductors.

5. Apparatus as defined in clairn 2 wherein said electrode is a metal conductor coated with cement of the type used in dentistry.

6. Apparatus as defined in claim 2 wherein said multiplier resistor means includes a 'fixed component.

7. Apparatus as defined in claim 2 wherein a switch is connected in series with said means to vary the voltage drop.

8. Apparatus as defined in claim 3 wherein a switch is connected in series with said mercury battery.

References Cited in the le of this patent UNITED STATES PATENTS 1,962,696 Rhodes June 12, 1934 2,183,531 Allison Dec. 19, 1939 2,344,672 Blasier Mar. 21, 1944 2,371,658 Stewart Mar. 20, 1945 2,556,705 Pike June 12, 1951 2,694,179 Walstrorn Nov. 9, 1954 

